Highly corrosion-resistant copper tube

ABSTRACT

A copper tube which can exhibit an improved resistance against ant nest corrosion, and which is suitably usable in air-conditioning equipment and refrigerating equipment, to advantageously extend a service life of the equipment produced by using the copper tube. The copper tube contains 0.05-1.0% by weight of P (phosphate) and the balance consisting of Cu (copper) and inevitable impurities.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of the International Application No.PCT/JP2014/052418 filed on Feb. 3, 2014, which claims the benefit under35 U.S.C. §119(a)-(d) of Japanese Application No.2013-055963 filed onMar. 19, 2013, the entireties of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a highly corrosion-resistant coppertube, and more particularly relates to a copper tube suitably usable asa heat transfer tube and a refrigerant tube in air-conditioningequipment and refrigerating equipment, for example.

2. Description of Related Art

A tube made of a phosphorous deoxidized copper (JIS-H3300-C1220T) havingexcellent properties in terms of corrosion resistance, brazability, heatconductivity and bending workability, for example, has been mainly usedas the heat transfer tube in the air-conditioning equipment, therefrigerant tube in the refrigerating equipment and the like.

However, it is recognized that the above-described phosphorousdeoxidized copper tube used in the air-conditioning equipment and therefrigerating equipment suffers from generation of so-called “ant nestcorrosion” (or “formicary corrosion”) which is an unusual corrosion thatprogresses in the form of an ants' nest from the surface of the tube ina direction of its wall thickness. The ant nest corrosion is consideredto be generated in a damp environment by a corrosive medium in the formof a lower carboxylic acid such as a formic acid and an acetic acid.Further, it is recognized that such corrosion is also generated in thepresence of a chlorine-based organic solvent such as1,1,1-trichloroethane, particular kinds of lubricating oil, andformaldehyde, for example. It is known that generation of the ant nestcorrosion is particularly remarkable where the phosphorous deoxidizedcopper tube is used as a conduit in the air-conditioning equipment andthe refrigerating equipment, which conduit is liable to dewing. Once theant nest corrosion is generated, it progresses rapidly and penetratesthrough the wall of the copper tube in a short time, giving rise to aproblem that the equipment becomes unworkable.

Under the above-described circumstances, JP-A-6-122932 proposes acorrosion-resistant high-strength copper tube, and discloses thatresistance of the copper tube against the ant nest corrosion is improvedsince the copper tube contains 0.0025-0.01 wt % of phosphate (P) and thebalance consisting of Cu and conventionally contained impurities, whichcopper tube may have an oxygen concentration not higher than 20 wtppm.Namely, based on the fact that generation of the ant nest corrosion isreduced in an oxygen-free copper tube having an extremely low P content,the P content of the copper tube proposed in the above-indicatedpublication is reduced as compared with that of the phosphorousdeoxidized copper tube, so that the copper tube has a higher resistanceagainst the ant nest corrosion than the phosphorous deoxidized coppertube.

However, the copper tube obtained by reducing the P content has not yetachieved a sufficiently high resistance against the ant nest corrosionwhich is comparable to that of the oxygen-free copper tube. Therefore,it is desired to develop a copper tube which can exhibit a higherresistance against the ant nest corrosion than the conventional coppertube, even in a severe corrosive environment.

SUMMARY OF THE INVENTION

The present invention was made in view of the background art describedabove. It is therefore an object of the invention to provide a coppertube which can exhibit a higher resistance against the ant nestcorrosion, and which has an excellent anti-corrosion property and whichis suitably usable in the air-conditioning equipment and therefrigerating equipment. Another object of the invention is toadvantageously extend a service life of equipment produced by using sucha copper tube.

The inventors of the present invention made intensive studies on the antnest corrosion generated in the copper tube used in the air-conditioningequipment, the refrigerating equipment and the like, and found that thecopper tube having a higher resistance against the ant nest corrosioncan be practically and advantageously obtained by setting the P contentof the copper tube to be higher than that of the conventionalphosphorous deoxidized copper tube. The present invention was completedbased on this finding.

Based on the above-described finding, the present invention provides aheat transfer tube used in a damp environment in air-conditioningequipment and exposed to a corrosive action caused by a corrosive mediumconsisting of a lower carboxylic acid, which corrosive action progressesin the form of an ants' nest from a surface of the heat transfer tube ina direction of its wall thickness, wherein the heat transfer tube is ahighly corrosion-resistant copper tube having a high resistance againstant nest corrosion and comprising 0.10-1.0% by weight of P (phosphate)and the balance consisting of Cu and inevitable impurities. The presentinvention also provides a refrigerant tube used in a damp environment inrefrigerating equipment and exposed to a corrosive action caused by acorrosive medium consisting of a lower carboxylic acid, which corrosiveaction progresses in the form of an ants' nest from a surface of therefrigerant tube in a direction of its wall thickness, wherein therefrigerant tube is a highly corrosion-resistant copper tube having ahigh resistance against ant nest corrosion and comprising 0.10-1.0% byweight of P and the balance consisting of Cu and inevitable impurities.

In the present invention, the P content of the highlycorrosion-resistant copper tube is set within a range of 0.10-1.0% byweight, which P content is higher, by a predetermined amount, than thatof the conventional phosphorous deoxidized copper tube, which is withina range of about 0.015-0.040% by weight. By using the above-describedhighly corrosion-resistant copper tube as the heat transfer tube forair-conditioning equipment and the refrigerant tube for refrigeratingequipment, the resistance of the heat transfer tube against the ant nestcorrosion and the resistance of the refrigerant tube against the antnest corrosion are significantly improved. It is particularly surprisingthat the heat transfer tube and the refrigerant tube according to thepresent invention have a higher resistance against the ant nestcorrosion than the conventional oxygen-free copper tube.

In a preferable form of each of the heat transfer tube forair-conditioning equipment and the refrigerant tube for refrigeratingequipment according to the invention, each of the heat transfer tube andthe refrigerant tube comprises not lower than 0.15% by weight of P.

In further preferable form of each of the heat transfer tube forair-conditioning equipment and the refrigerant tube for refrigeratingequipment according to the invention, each of the heat transfer tube andthe refrigerant tube comprises not higher than 0.8% by weight of P. Inother preferable form of the heat transfer tube for air-conditioningequipment and the refrigerant tube for refrigerating equipment accordingto the invention, each of the heat transfer tube and the refrigeranttube comprises not higher than 0.5% by weight of P.

In still other preferable form of each of the heat transfer tube forair-conditioning equipment and the refrigerant tube for refrigeratingequipment according to the invention, a total amount of the inevitableimpurities is not higher than 0.05% by weight.

The present invention also provides a method for improving a corrosionresistance of a copper tube used in a damp environment inair-conditioning equipment or refrigerating equipment, against ant nestcorrosion which is generated in the damp environment by a corrosivemedium consisting of a lower carboxylic acid, and progresses from asurface of the copper tube, wherein the resistance of the copper tubeagainst the ant nest corrosion is improved by forming the copper tube byusing a material comprising 0.10-1.0% by weight of P and the balanceconsisting of Cu and inevitable impurities. In a preferable form of themethod for improving the corrosion resistance of the copper tubeaccording to the invention, the material of the copper tube comprises0.3-1.0% by weight of P. In other preferable form of the method forimproving the corrosion resistance of the copper tube according to theinvention, a total amount of the inevitable impurities contained in thematerial of the copper tube is not higher than 0.05% by weight.

According to the present invention, the practical copper tube which issuperior to the conventional copper tube in its anti-corrosion propertyin terms of the resistance against the ant nest corrosion can beprovided. Further, by using the copper tube according to the presentinvention as the heat transfer tube in the air-conditioning equipmentand the refrigerant tube in the refrigerating equipment, the servicelife of those equipment can be extended.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view showing an apparatus used fora corrosion resistance test in illustrated Examples; and

FIG. 2 is a graph showing a relationship between a P content of coppertubes obtained in the Examples and a maximum corrosion depth of thosecopper tubes.

DETAILED DESCRIPTION OF THE INVENTION

A highly corrosion-resistant copper tube according to the invention hasa major characteristic that its phosphate (P) content is held within arange of 0.05-1.0% by weight and higher than that of the conventionalcopper tube. It is considered that owing to such a high P content, thetype of corrosion generated in the copper tube shifts from a selectivecorrosion which progresses in a direction perpendicular to the axialdirection of the copper tube (i.e., in a direction of the wall thicknessof the copper tube) to a surface corrosion which progresses in adirection parallel to the axial direction of the copper tube (i.e., in adirection extending along the surface of the copper tube). Inparticular, by setting the P content of the copper tube so as to be notlower than 0.10% by weight, and preferably not lower than 0.15% byweight, generation of the selective corrosion is effectively reduced orprevented, and the copper tube can exhibit corrosion resistance which isconsiderably higher than that of the conventional copper tube.

Where the P content of the copper tube is as low as 0.05% by weight, theselective corrosion is generated, but a rate of progress of theselective corrosion in the copper tube can be effectively reduced ascompared with that in the conventional copper tube, so that the coppertube is recognized to have a higher resistance against the ant nestcorrosion. Therefore, the P content of the copper tube is set so as tobe not lower than 0.05% by weight, in the present invention. On theother hand, the upper limit of the P content of the copper tube needs tobe set at 1.0% by weight, since the P content higher than 1.0% by weightcauses almost no change in the resistance of the copper tube against theant nest corrosion, and even causes deterioration of workability of thecopper tube during its production, giving rise to a problem of crackingof the copper tube, for example. From the standpoint of practicalproduction of the copper tube, the P content of the copper tube ispreferably set so as to be not higher than 0.8% by weight, and morepreferably not higher than 0.5% by weight.

The highly corrosion-resistant copper tube according to the presentinvention is made of a material having the P content described abovewith the balance consisting of Cu (copper) and inevitable impurities. Atotal amount of the inevitable impurities such as Fe, Pb and Sncontained in the copper tube is generally controlled so as to be nothigher than 0.05% by weight.

The intended copper tube is produced by using a Cu material having theabove-described composition according to the invention, by a methodsimilar to the conventional method. For example, the copper tube isproduced by steps of casting an ingot or a billet, and extruding anddrawing the ingot or billet. Dimensions such as the outside diameter andthe wall thickness of the thus obtained copper tube are adequatelydetermined depending on the intended application of the copper tube.Where the copper tube according to the invention is to be used as a heattransfer tube, the copper tube may have a smooth internal surface, ormay advantageously have various kinds of internal grooves formed in itsinternal surface by various known processes, as is well known in theart.

EXAMPLES

To clarify the present invention more specifically, some examplesaccording to the present invention will be described. It is to beunderstood that the invention is by no means limited by the details ofthe illustrated examples, but may be embodied with various changes,modifications and improvements which are not described herein, and whichmay occur to those skilled in the art, without departing from the spiritof the invention.

Initially, various kinds of copper tube having compositions includingrespective P contents indicated in Table 1 given below, with the balanceconsisting of Cu and inevitable impurities were produced as inproduction of the conventional copper tube, such that each copper tubehas an outside diameter of 9.52 mm and a wall thickness of 0.41 mm. Thethus produced copper tubes were subjected to an ant nest corrosion test,as described below. Further, a Cu material containing 1.5% by weight ofP and the balance consisting of Cu and inevitable impurities was used toproduce a copper tube having dimensions similar to those of theabove-described copper tubes, but the intended copper tube could not beobtained due to cracking of the tube. A phosphorous deoxidized coppertube and an oxygen-free copper tube each having the same dimensions asthose of the above-described copper tubes were provided as comparativecopper tubes.

TABLE 1 Copper tube P content No. Kind of copper tube (% by weight) 1Copper tube according to the invention 0.11 2 Copper tube according tothe invention 0.19 3 Copper tube according to the invention 0.30 4Copper tube according to the invention 0.40 5 Copper tube according tothe invention 0.50 6 Copper tube according to the invention 1.00 7Phosphorous deoxidized copper tube 0.03 8 Oxygen-free copper tube <0.004

Each of the thus provided various kinds of copper tube was subjected tothe ant nest corrosion test by using a test apparatus shown in FIG. 1. Aplastic container 2 shown in FIG. 1 has a capacity of 2 L and can behermetically sealed with a cap 4. Silicone plugs 6 are attached to thecap 4 such that the plugs 6 extend through the cap 4. Copper tubes 10 tobe subjected to the corrosion test were inserted into the plasticcontainer 2 by a predetermined length, such that the copper tubes 10extend through the respective silicone plugs 6. Lower open ends of thecopper tubes 10 were dosed with silicone plugs 8. 100 mL of a formicacid aqueous solution having a predetermined concentration wasaccommodated in the plastic container 2, such that the copper tubes 10do not contact with the aqueous solution.

The ant nest corrosion test was conducted by using three kinds of formicacid aqueous solutions 12 having respective concentrations of 0.01%,0.1% and 1%. The copper tubes 10 were set with respect to each of theplastic containers 2 in which the respective formic acid aqueoussolutions 12 were accommodated, and the plastic container 2 was leftwithin a constant temperature bath at a temperature of 40° C. Theplastic container 2 with the copper tubes 10 was taken out of the bathfor two hours each day, and held at the room temperature (15° C.), tocause dewing on surfaces of the copper tubes 10 by the differencebetween the temperature of the constant temperature bath and the roomtemperature. The copper tubes 10 were subjected to the corrosion testunder the above-described conditions for 20 days.

Each of the copper tubes subjected to the corrosion test using each ofthe formic acid aqueous solutions having the respective concentrationswas examined in its cross section, and measured of its maximum corrosiondepth. Results of the measurement are indicated in Table 2 given below.Further, a relationship between the maximum corrosion depth of thecopper tubes subjected to the corrosion test using the 0.1% formic acidaqueous solution and the P content of the respective copper tubes isindicated in a graph of FIG. 2.

TABLE 2 Copper Maximum corrosion depth (mm) tube Formic acid Formic acidFormic acid No. concentration: 0.01% concentration: 0.1% concentration:1% 1 0.10 0.25 — 2 0.06 0.11 0.12 3 0.08 0.04 0.08 4 0.04 0.05 0.07 50.03 0.04 0.10 6 <0.03 0.05 — 7 0.15 0.40 >0.40 8 0.05 0.30 >0.40

As is apparent from the results in Table 2, in the corrosion testconducted by using the formic acid aqueous solution having theconcentration of 0.01%, the ant nest corrosion was not generated andonly slight corrosion on the surfaces of the copper tubes was recognizedin the copper tubes Nos. 1-6 having P contents within a range of0.1-1.0% by weight, and the copper tube No. 8 which is the oxygen-freecopper tube. On the other hand, in the corrosion test conducted by usingthe formic acid aqueous solutions having the respective concentrationsof 0.1% and 1%, the ant nest corrosion was recognized in both of thecopper tube No. 7 which is the phosphorous deoxidized copper tube, andthe copper tube No. 8 which is the oxygen-free copper tube, andcorrosion was recognized in the copper tubes Nos. 1-6 having the Pcontents within the range of 0.1-1.0% by weight. However, the corrosiongenerated in the copper tubes Nos. 1-6 was not the ant nest corrosion,and maximum corrosion depths of the copper tubes Nos. 1-6 are smallerthan those of the phosphorous deoxidized copper tube and the oxygen-freecopper tube.

Further, as indicated in FIG. 2, the copper tubes having P contentshigher or lower than the P content of 0.03% by weight of the phosphorousdeoxidized copper tube (No. 7) have smaller maximum corrosion depthsthan the phosphorous deoxidized copper tube (No. 7). It is particularlynoted that the copper tubes (Nos. 1-6) according to the presentinvention having higher P contents than the phosphorous deoxidizedcopper tube (No. 7) are superior to the oxygen-free copper tube (No. 8)in their maximum corrosion depths.

1. A heat transfer tube used in a damp environment in air-conditioningequipment and exposed to a corrosive action caused by a corrosive mediumconsisting of a lower carboxylic acid, which corrosive action progressesin the form of an ants' nest from a surface of the heat transfer tube ina direction of its wall thickness, wherein the heat transfer tube is ahighly corrosion-resistant copper tube having a high resistance againstant nest corrosion and comprising 0.10-1.0% by weight of P and thebalance consisting of Cu and inevitable impurities.
 2. The heat transfertube according to claim 1, comprising not lower than 0.15% by weight ofP.
 3. The heat transfer tube according to claim 1, comprising not higherthan 0.8% by weight of P.
 4. The heat transfer tube according to claim1, comprising not higher than 0.5% by weight of P.
 5. The heat transfertube according to claim 1, wherein a total amount of the inevitableimpurities is not higher than 0.05% by weight.
 6. A refrigerant tubeused in a damp environment in refrigerating equipment and exposed to acorrosive action caused by a corrosive medium consisting of a lowercarboxylic acid, which corrosive action progresses in the form of anants' nest from a surface of the refrigerant tube in a direction of itswall thickness, wherein the refrigerant tube is a highlycorrosion-resistant copper tube having a high resistance against antnest corrosion and comprising 0.10-1.0% by weight of P and the balanceconsisting of Cu and inevitable impurities.
 7. The refrigerant tubeaccording to claim 6, comprising not lower than 0.15% by weight of P. 8.The refrigerant tube according to claim 6, comprising not higher than0.8% by weight of P.
 9. The refrigerant tube according to claim 6,comprising not higher than 0.5% by weight of P.
 10. The refrigerant tubeaccording to claim 6, wherein a total amount of the inevitableimpurities is not higher than 0.05% by weight.
 11. A method forimproving a corrosion resistance of a copper tube used in a dampenvironment in air-conditioning equipment or refrigerating equipment,against ant nest corrosion which is generated in the damp environment bya corrosive medium consisting of a lower carboxylic acid, and progressesfrom a surface of the copper tube, wherein the resistance of the coppertube against the ant nest corrosion is improved by forming the coppertube by using a material comprising 0.10-1.0% by weight of P and thebalance consisting of Cu and inevitable impurities.
 12. The method forimproving the corrosion resistance of the copper tube according to claim11, wherein the material of the copper tube comprises 0.3-1.0% by weightof P.
 13. The method for improving the corrosion resistance of thecopper tube according to claim 11, wherein a total amount of theinevitable impurities contained in the material of the copper tube isnot higher than 0.05% by weight.